Editor: Merge latest fluid typography bugfixes for 6.1 Release Candidate 1.

Merges the following:
* [https://github.com/WordPress/gutenberg/pull/44761 Gutenberg PR 44761] - Fluid Typography: Fix bug in global styles where fluid `clamp()` rules were not calculated for custom values
* [https://github.com/WordPress/gutenberg/pull/44762 Gutenberg PR 44762] - Fluid Typography: Convert server-side block support values
* [https://github.com/WordPress/gutenberg/pull/44764 Gutenberg PR 44764] - Fluid Typography: Convert font size inline style attributes to fluid values
* [https://github.com/WordPress/gutenberg/pull/44807 Gutenberg PR 44807] - Fluid Typography: Covert font size number values to pixels
* [https://github.com/WordPress/gutenberg/pull/44847 Gutenberg PR 44847] - Fluid Typography: ensure font sizes are strings or integers

Follow-up to [54280].

Props andrewserong, isabel_brison, ramonopoly.
See #56467.

git-svn-id: https://develop.svn.wordpress.org/trunk@54497 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-theme-json.php

@@ -1675,6 +1675,18 @@ class WP_Theme_JSON {
 				continue;
 			}
 
+			// Calculates fluid typography rules where available.
+			if ( 'font-size' === $css_property ) {
+				/*
+				 * wp_get_typography_font_size_value() will check
+				 * if fluid typography has been activated and also
+				 * whether the incoming value can be converted to a fluid value.
+				 * Values that already have a clamp() function will not pass the test,
+				 * and therefore the original $value will be returned.
+				 */
+				$value = wp_get_typography_font_size_value( array( 'size' => $value ) );
+			}
+
 			$declarations[] = array(
 				'name'  => $css_property,
 				'value' => $value,